Lot Tracking and RMA

We just turned on Lot Tracking.

Everything is OK- We entered the Lots on existing Customer Shipments.

Now, we are trying to ship an RMA and the Lot button is greyed out but, we can choose next lot which is 1 - but, not our lot.

I went back to the RMA and the lot button was greyed out but, we can choose next lot which is not our lot number.

How do I fixed this?

Actually - Next lot is not highlighted in Customer Shipment entry.

Actually fund the problem - we were putting Warranty as a part number on the Sales Order - the RMA job had a different PN.